1. Field of the Invention
This invention relates to switches for electric power systems and more particularly to an isolation switch for medium-voltage switchgear.
2. Background Information
Electric power systems include switchgear that distribute power from source buses to load buses and typically provide protection for the load buses. Isolation switches allow the downstream devices to be disconnected from the source bus, such as for maintenance, and provide the capability of connecting the de-energized load bus to ground to protect those working on the system.
A common type of isolation switch has three poles, each including a straight copper conductor mounted for rotation about a transverse axis through the mid point of the conductor with the three pole conductors axially spaced along the common axis of a support shaft. A fixed line contact and a fixed load contact for each pole are positioned in a housing diametrically opposite one another for engagement with the two ends of the movable conductor with the main shaft in a xe2x80x9cconnected xe2x80x9d position to provide electrical continuity between the feeder line and the load bus. A second load contact and a ground contact for each pole are positioned diametrically opposite one another 90xc2x0 from the diametrically opposite fixed line contact and first load contact for engagement by the two ends of the movable conductor with the shaft in a xe2x80x9cgroundedxe2x80x9d position to connect the load bus to ground. In these typically medium voltage isolation switches, the poles must be sufficiently spaced axially to prevent arcing and multiple fins are provided on the insulative covers on the movable conductors to provide the required creep distance from the ends of the movable conductors to the metal shaft.
There is room for improvement in isolation switches for electric power systems.
This need and others are satisfied by the invention which is directed to an isolation switch for electric power circuits which includes a housing, a shaft mounted for rotation about its longitudinal axis within the housing and one or more pole units each comprising a movable conductor carried by the shaft and having a first movable contact at one end and a second movable contact at another end. The first and second movable contacts are angularly spaced in a plane perpendicular to the longitudinal axis of the shaft by an angle xcex1. The isolation switch in accordance with the invention further includes a fixed load contact, a fixed line contact and a fixed ground contact all mounted in the housing in the plane perpendicular to the main shaft. The fixed load terminal is disposed between and angularly spaced from the fixed line contact and the fixed ground contact by the angle xcex1. The shaft is rotatable to a first position in which the first movable contact engages the fixed load contact and the second movable contact engages the fixed line contact, and a second position xcex1 degrees from the first position in which the first movable contact engages the fixed ground contact and the second movable contact engages the fixed load contact.
In accordance with another aspect of the invention, the isolation switch comprises a housing, an elongated electrically insulative shaft with a metallic core extending along a longitudinal axis about which the shaft is mounted in the housing for rotation. The isolation switch includes one or more pole units each comprising a movable conductor embedded in and solely supported by the elongated electrically insulative shaft in electrical isolation from the metallic core in a plane substantially perpendicular to the longitudinal axis of the shaft. The shaft is rotatable between a connected position in which the movable conductor connects the load conductor of the electrical system to the line conductor, and a grounded position in which the movable conductor connects the load conductor to the ground conductor.